Thursday, November 15, 2007

One of the coolest things about astronomy is that we live in a universe. A big universe. And that means that there's a lot of fun stuff to find out there. It's like a never ending easter egg hunt. Unfortunately, these eggs don't contain chocolate. But they do glow in the dark, so that's pretty cool.

What's especially amazing sometimes, is that we are still trying to catalog our own galactic back yard. Phil Plait at Bad Astronomy addressed the reason for this (transcript), namely that things are pretty darn faint. So, even in our own galaxy, we can't really see terribly far.

Break out the telescopes, and many more things become visible. But then you start to run into some other problems. First off, we live in a spiral galaxy. Spiral galaxies are partially known for having large amounts of dust and gas. So it's a lot like trying to look around in a fog bank. You can't see too far. Another problem is that the further away things are, the smaller they are in angular size. So even if something is relatively bright, you might not even notice it in all the clutter.

Historically, new objects were discovered just wandering around the sky with telescopes and documenting anything that was suspicious. This was the foundation for the famous Messier Catalog. Now, serendipitous discoveries along those lines don't happen too often, although, they occasionally still do.

Today, instead of wandering aimlessly across the sky, new discoveries are often the result of computer controlled survey missions. Sometimes the astronomers know what they're looking for. Other times, new discoveries are simply a byproduct of other data taking. An example of the latter is the numerous comets discovered by the SOHO mission which has discovered over 1,300 comets (including it's first perioditic comet recently).

Another example is the Sloan Digital Sky Survey (SDSS), which slowly scans the sky, taking high resolution images with its 120 megapixel camera. Sloshing through this data, a group from the Max Plank Institute with Cambridge astronomers have discovered 2 new globular clusters in our galaxy (Kopsov, 2007). To do this, a computer program mapped the stars imaged in the survey and looked for places where there were spikes in the density of them. But a jump in the density of stars alone doesn't tell astronomers what the object is.

Since not only our universe is big, but we also live in a pretty good sized galaxy, our galaxy gets to be a bit of a bully. This means we cannibalize dwarf galaxies. So when we look around some of the overdensities can also be the result of these tiny doomed galaxies.

So how can we tell the difference? One of the main ways is to look at their HR diagrams. If it's a cluster, all stars will have formed at the same time, and thus, you should see something resembling an isochrone. If it's a galaxy, then stars would have formed at largely different times and there should be no distinct turn off. Since the color-magnitude diagrams (CMDs, essentially an HR diagram) show distint main sequence turn offs, the possibility that these are distinct galaxies can be ruled out.

Now that we know that these are clusters, what can we say about them. One of the first things that the paper notes is that these clusters are faint. Each is estimated to have a total surface brightness of -2 to -1 magnitudes (remember, in the magnitude system, bigger numbers are fainter. Thus, a -20m object is brighter than a -15m object. Similarly, a 1m object is brighter than a 2m object.) To date, the faintest globular clusters ever found had magnitudes of roughly -1.6.

Additionally, these two new clusters are way out there in the halo of the galaxy. Finding faint galaxies that far out hints that there may be a lot more of these bitty guys out there that have yet to be discovered. So why are they so puny while most of the inner globulars are nice and big?

It looks like these clusters are falling apart due to the tidal stripping that keeps coming up in my posts. This would suggest that in the past, there were even more of these and they were bigger. But fear not creationists. The current estimated lifetime for clusters with this density is about 8 billion years. So the evaporating comet nonsense doesn't work for these objects either.

But globular clusters aren't the only objects lurking in our galaxy that are yet to be discovered. In another paper from the same ApJ issue discusses some more findings from another sky survey. This one was looking for planetary nebulae (Cohen, 2007). These guys don't last all that long before they spread out and blend back into the interstellar medium. But since they're the result of low mass stars ending their lives and there's a good number of stars doing that, there should be a pretty good number out there.

However, since these form from dying stars, we should expect more where there are stars, namely, in the disk of the galaxy. So unlike globular cluster hunting, we have to look close to the galactic plane. Unfortunately, this means there's going to be a lot of extra dust and gas in the way hiding them.

The Macquarie-AAO-Strasbourg Hα PN project (MASH) again uses sky surveys to look for these objects. Instead of looking for density spikes, they look for objects with emission due to hydrogen and then compare the brightness of that in the mid-infrared part of the spectrum, to the brightness in the radio as well as differences in spectra to distinguish the objects from other objects that may masquerade as planetary nebulae (such as HII regions).

So far, this study has found 905 new objects that are likely to be planetary nebulae in our own galaxy! This has increased the known number by ~60%. Wow! This paper alone added an additional 58 to that list. Many of these new nebulae are quite evolved and starting to diffuse back into the interstellar medium, so having this large collection of additional objects will allow us to put new constraints on these objects and figure out how they evolve.

Another thing this survey gives us is a lot of new PNe morphologies to study. Ideally, they should all be nice and symmetrical (since stars are round and all). But the universe is never idea (*sigh*). Stellar rotation, magnetic fields, asymmetrical releases, companion stars and all sorts of nasty things whip planetary nebulae into odd shapes. There's so many ways to do things that it's hard to untangle them all and hopefully, having more to study will untangle the mess. Roughly 28% of these new PNe are bipolar which is a pretty significant number.

So now we've seen two examples of how large sky surveys are uncovering scores of new objects that can help us learn about the universe we live in. Nice little benefit of these things that is, but there's also a downside. Having gigantic surveys like this generates loads of data. Way more than astronomers can possibly sort through. That's why we've been seeing projects that are starting to put some of the grunt work in the hands of passionate amateurs. Hopefully we'll develop more sophisticated ways of handling the terrabytes of data that can be generated in a single night of observation.Koposov, S., de Jong, J.T., Belokurov, V., Rix, H., Zucker, D.B., Evans, N.W., Gilmore, G., Irwin, M.J., Bell, E.F. (2007). The Discovery of Two Extremely Low Luminosity Milky Way Globular Clusters. The Astrophysical Journal, 669(1), 337-342. DOI: 10.1086/521422